Right-Mouse Button Method - Voila! - Straightened Picture

As you move the mouse (with the right mouse button depressed), you will see the picture rotate.  You will have to judge when the photo is "straight", but there you go.  When it looks straight, just let the mouse button loose.  You can depress it, and adjust it again, if ou need to. 

Note we have the problem with the "Corners" again.  This time, we will fix it by "Cropping" the photo.    First, locate the "Tools" bar.